The present invention relates generally to power utilization and more particularly to a method and apparatus for controlling power utilization in a power sub-network of a power grid based on external global power parameters.
Large-scale electrical power distribution occurs over what is known in the industry as a power grid. More particularly, a power grid is an expansive network of high-voltage power transmission lines interconnected at hundreds of power generating stations and distribution substations. The stations and substations are typically owned by various utility companies, which manage distribution of power to various sectors of the grid, including distribution of power to consumers over lower-voltage power lines that are stepped-down via transformers. The substations essentially operate as power hubs, directing current from branches with power surplus to branches needing additional power. The power substations thus operate to attempt to equalize distribution of power across the sectors of the grid and to allow the utilities to buy and sell electricity from each other. Substations may also transform some of the power to a lower voltage level and direct it onto lower-voltage distribution lines that service local sectors. End consumers in the local sectors are fed through service lines that are connected to the lower-voltage distribution lines.
Ohm's law is a basic law of the relationship between voltage, resistance, and current. The relationship can be expressed as I=V/R (or current=voltage divided by resistance). This means that there is an inverse relationship between resistance and current, assuming the voltage is held constant. In a discussion of how electricity is delivered from the electrical power grid to the consumers of that electricity, it is a fair statement to say that it is the intent of the electrical utility to provide a constant voltage for the electrical service delivered to the consumers, and as much current as consumers desire. It is also true that the consumers dictate the quantity of current being drawn at the intended voltage by varying the load (1/resistance) that the overall power grid experiences through the switching on or off of electricity consuming devices such as lamps and motors. As a result, in view of the fact that voltage is intended to be fixed by the utilities and that consumers dictate the load based upon their usage, the current being drawn can be viewed as the dependent variable in this relationship.
The electrical power grid is designed to be a balanced system encompassing a number of power providers whose contributions vary as necessary in order to meet the overall current requirements of the electricity consumers. The grid or transmission infrastructure can be viewed, for the purposes of this patent, as a number of parallel transmissions lines linking the electricity producers and consumers, and over which the current is carried. For the purposes of this patent, the transmission lines refer to all of the components that are not part of the electricity producer, not part of the electricity consumer, and that are necessary for carrying current between the producers and consumers.
By design, these adjacent or effectively parallel transmission lines divide and balance the carrying of the current necessary to satisfy the consumers load, or demand, such that no individual transmission line is damaged by attempts to carry an amount of current in excess of its current carrying capacity.
There are typically two approaches taken by the utility service to react to a situation where a transmission line is experiencing excessive current. The first approach is to employ a current limiting device in series with the system load. These devices typically react to increases in heat from the overloading transmission line and become increasingly resistive to current flow as the temperature of the transmission line increases. Use of devices like these limits the current being carried over the transmission line, but results in either parallel transmission lines having to carry more current to meet effective demand, or in the consumer experiencing decreased voltage at their location. Decreased voltage at the consumer's site is unacceptable since it can damage voltage sensitive equipment that is expecting the voltage of the electrical service to remain within reasonably tight acceptable limits.
The second option for reacting to excessive current on a transmission line is to disconnect the transmission line that is overloading via the use of a manual or automated control device before damage to the transmission line can occur. Unfortunately this approach has two negative side effects. First, when one of the parallel transmission lines between the producers and the consumers of electrical current is disconnected, whether intentionally or due to a fault, it places a greater current burden on the remaining parallel transmissions lines. This additional current is know as a fault-current, and follows from Ohm's law which dictates that the full current required to satisfy the demand still be delivered (assuming voltage is to be held constant), and is therefore divided over the remaining transmission lines as necessary to achieve this. In this scenario, each of the parallel transmission lines will now be carrying an amount of current that is greater than the current it carried prior to the fault having occurred—thus, the proportion of the total current that each remaining transmission line carries increases.
As the parallel lines near the point of overloading, automated controllers disconnect them from the grid, increasing the number of fault-current loads on other lines exponentially. If not controlled properly, disconnecting transmission lines that are overloaded can thus result in a cascading failure of the entire transmission system as the same total current is attempted to be carried over an increasingly reduced number of available transmission lines.
The objective of shutting down an overloaded transmission line is two-fold. The first objective is to protect the transmission line and its components from damage resulting from the excessive current and resultant heat. The second objective is to attempt to disconnect a load from the grid by shutting down a transmission line, following the reasoning that disconnection of the load from the electrical grid will reduce current demand sufficiently to satisfy the remaining load connected to the grid. This is a pruning of the power demand in order to reduce the current to within levels that the transmission lines can support.
However, this approach to addressing the problem suffers from the fact that the granularity at which the electrical grid can prune itself is extremely coarse. Typically the granularity is at the neighborhood or regional scale—thus a whole neighborhood or region is shut down simultaneously. In the case of resulting cascading failures such as during the blackouts of 2003 in the U.S., the scale of the blackouts became much greater.
There are two fundamental ways to address the issue of excessive current through parts of the transmission system. The first is to increase the current carrying capability of the transmission system. This approach faces several challenges including resistance at the state and local regulatory levels, significant expense, and difficulty gaining right of way for expansion of the transmission system.
The second approach is to reduce the electrical demand during times of excessive current draw. Reducing demand proportionally reduces the current required, and reduces the risk of damage from excessive current on the existing transmission system. These approaches are not mutually exclusive. Voluntary reductions in demand are attempted through the use of “realtime pricing” or “demand pricing”. With demand pricing, there is a financial disincentive to using power during high use periods. Clearly this approach can, at best, only have a probabilistically positive effect on current draw, and at the present time could not be depended upon as a means to control current draw during an unforeseen incident such as that of the blackouts of 2003.
Accordingly, it would be desirable to have a technique for enabling the control of electrical demand at a much finer granularity and in a much more selective way, than previously possible. As a result, during an electrical disturbance, operators of the electrical grid would be able to request the reduction of electrical load as necessary to reduce the current on the transmission lines in an automated way with the result being that load is reduced according to the consumers' preferences for what is shutdown.
It would also be desirable to have a technique for allowing electricity consumers to fully take advantage of the increasing trend in “realtime electricity pricing”, or “demand electricity pricing”. In realtime or demand electricity pricing, the price for electrical power is adjusted many times over during the course of the day to better reflect the true cost of electricity production and transmission rather than being set less frequently based on averaging models. The advantage of this approach would be that it allows consumers to adjust their consumption in response to changes in pricing. However, in order to take full advantage of this approach, the consumers need to be made aware of these price changes, and to have a means for quickly adjusting their consumption in response to the changes in price. This invention allows consumers to respond in an automated fashion to rapid changes in the price of electricity. This ability provides potentially significant savings for consumers, and can significantly reduce the peak electricity demand as seen by the electricity producing utilities. If peak electricity demand is reduced, then the current carrying capabilities of the electricity transmission system are also reduced.
Separately, the utility companies, recognizing the existence of a market for providing fast Internet services to households, are endeavoring to provide Internet service using the power system wiring. To achieve this, several technologies have been invented, referred to as Broadband Power Line (BPL) technologies, which use the existing Utility Service power lines to provide a data communication channel that can be used to provide Internet access to a customer. BPL technologies are primarily intended to be inter-premises technologies. They are designed to provide Internet access to the home or business, in the same way that DSL and Cable networking companies currently do.
Separately, intra-home networking is popular, but prior to the availability of wireless networking, required wiring the premises for data using one of several cable standards that are well known to someone skilled in the art.
Existing products are available for utilizing the existing premises electricity wiring for the purposes of home networking. Some of these products are based on the HomePlug standard. Products are available that allow one to bridge Ethernet communications into the HomePlug network which utilizes the electrical wires in the house. These bridges are plugged into a wall outlet, and provide a standard 10-Base-T or 100-Base-T RJ-45 connector to which a computer can be connected. By utilizing two of these devices, the computers are networked, with the communications path taking advantage of the existing in-house, or “premises”, wiring. HomePlug enables the use of the premises electrical wiring to allow a communications network within the premises electrical network.